Recently, there has been a rapidly increasing importance of an optical pulse shaper for generating and outputting ultra-short optical pulses.
In the area of optical communications, with use of a system in which a bit rate of each channel exceeds 40 Gb/s, a repetition frequency is substantially high, and accordingly, optical pulses are required to have high quality, less noise, less jitter and the like. In addition, when a relay unit is inserted into an optical propagation line, it is required to generate local clock pulse trains for a function of optical signal regeneration to recover from waveform-deteriorated optical pulses caused during propagation. In other words, it is necessary to generate, under a high repetition frequency of more than 40 GHz, ultra-short optical pulse trains having an improved repetition characteristic, a pulse waveform of high quality and a very short pulse width.
Meanwhile, the area of optical processing makes use of a multi-photon absorption process using an optical pulse having a width of femtosecond order, and it is expected to realize a method of such processing that could not be obtained conventionally.
In addition, the conventional ultra-short pulse generating techniques are roughly classified into a technique using a cavity structure and a technique using no cavity structure.
In the ultra-short pulse generating technique using a cavity structure, a solid-state laser such as Ti: Sapphire Laser, a mode synchronization fiber laser having a cavity structure configured by the optical fiber itself, a semiconductor mode synchronization laser having a mode synchronization structure configured by a semiconductor or the like are used.
On the other hand, the ultra-short pulse generating technique using no cavity structure utilizes such a phenomenon that an optical pulse as a seed signal is compressed by the nonlinear effect in an optical fiber. As no cavity structure is used, the technique is called “traveling wave (TW) system”, and an optical soliton compressor, a super continuum compressor and the like are known.
According to the ultra-short pulse generating technique using a cavity structure, as the repetition frequency is determined by the cavity length, flexibility as to the repetition frequency is small. Further, for continuous pulse oscillation, various stabilization techniques are required and fine adjustment is required in accordance with change in the external environment.
On the other hand, according to the ultra-short pulse generating technique using no cavity structure (particularly, the ultra-short pulse generating technique based on the TW system using an optical fiber), as the width of an optical pulse, beat light or the like, which is a seed signal, generated from an electric circuit is compressed, the repetition frequency is tunable. Further, as no cavity structure is used, it is possible to output an optical pulse of extremely high stability in accordance with the stability of an optical fiber itself.
Here, known as the compression of an optical pulse width by TW system are SC compression and optical soliton compression.
SC compression is of compressing an optical pulse width by broadening the frequency band of the optical pulse by the nonlinear effect of an optical fiber of small dispersion and then compensating dispersion of the broadened frequency band. In order to perform this SC compression efficiently, it is preferable that the optical fiber dispersion is flat over frequencies and does not vary along the fiber longitudinal direction.
In the SC compression, a large compression ratio can be obtained, however there are problems of degradation of pulse quality and the like because of a pedestal component included in the width of a compressed optical pulse.
On the other hand, an optical soliton generated by optical soliton compression is an optical pulse which is formed by balancing of the dispersion effect and the nonlinear effect of an optical fiber and has a waveform not changed during propagation.
For optical soliton, when the dispersion effect is gently decreased in the optical fiber longitudinal direction for example by reducing a dispersion value for a sufficiently long span for the distance called soliton period, a pulse waveform is self-shaped so as to compensate the reduced dispersion effect, which results in an optical pulse of small width. Alternatively, when the pulse amplitude is gently increased in the optical fiber longitudinal direction by performing distribution amplification by Raman amplification while maintaining the optical fiber dispersion constant, the nonlinear effect is enhanced, and then, a pulse waveform is self-shaped so as to compensate this enhanced nonlinear effect, which result in an optical pulse of small width. The method using such a phenomenon (balancing of the dispersion effect and the nonlinear effect) is called adiabatic soliton compression and capable of outputting optical pulses as optical solitons of sech function.
Adiabatic soliton compression makes it possible to generate sech function shaped waveform, that is, optical pulse waveform of high quality having no pedestal, and therefore, it is suitable for communication purposes.
In addition, when an optical pulse with pedestal is compared with an optical pulse having no pedestal, if both of them has the same energy and FWHM (full width at half maximum), the optical pulse with no pedestal has larger peak power. Therefore, the optical pulse with no pedestal is suitable for material processing purposes.
Meanwhile, it is known the adiabatic soliton compression is, as described above, performed in a method of gently varying optical fiber dispersion along the fiber longitudinal direction. In this method, as it is not easy to manufacture an optical fiber having dispersion which decreases continuously in the fiber longitudinal direction, some other methods are contemplated.
For example, optical fiber dispersion is divided into several sections along the fiber longitudinal direction and fibers having fixed dispersions corresponding to the respective sections are concatenated to approach the decreasing dispersion. An optical fiber configured by this method is called “Step-like Dispersion Profiled Fiber (SDPF)”. According to this method, approximation accuracy is increased as the fiber has more steps (sections), however it is required to prepare many fibers of different dispersions.
As another method, it is contemplated to divide optical fiber dispersion into several sections along the fiber longitudinal direction and use two fibers of different dispersions in each of the sections to approximate the dispersion. An optical fiber configured by this method is called “Comb-like Dispersion Profiled Fiber (CDPF)” as the dispersion varies like comb in the fiber longitudinal direction (see Non-patent documents 1 and 2, Patent documents 1 and 2).    Non-patent document 1: S. V. Chernikov et al., “Integrated all optical fiber source of multigigahertz soliton pulse train”, Electronics Letters, 1993, vol. 29, p. 1788    Non-patent document 2: S. V. Chernikov et al., “Comblike dispersion-profiled fiber for soliton pulse train generation”, Optical Letters, 1994, vol. 19, no. 8, p. 539-541    Patent document 1: Japanese Patent Application Publication No. 2000-347228    Patent document 2: Japanese Patent Application Publication No. 2002-229080